I've attached a snap of the part I'm unable to understand Machinery's Handbook 29th Edition. Here's a link to the ebook as well https://www.vtc1.org/cms/lib/PA03000913/Centricity/Domain/21/Machinerys%20Handbook%2029th%20Edition.pdf
(page 1537)

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I'm struggling to understand the relevance of this value Q that I arrive on. It gives me the minimum thread engagement length but doesn't factor in the load on the internal/external material.

So I get the thread engagement length from just the material properties.

What if I had the same materials and the bolt was only bearing a 0.5 N load, and my engagement was only 5 mm. Will the threads still strip? I think this recommended length has some other significance. But I need someone's help in understand that.


1 Answer 1


The minimum length of engagement ist the length of engagement that (computationally) will ensure that overtightening the screw will snap the screw, and not strip the threads.

The external and internal threads (that are engaged) need to carry more load than the shaft, for this to happen, so both external and internal thread material properties come into play.

  • $\begingroup$ Thank you for the response. Is it possible I can get a source for this answer, convincing as it is. I'd like to read more about it and also be able to explain it to others if needed. $\endgroup$
    – Wren
    Commented Jan 3, 2022 at 10:48
  • 1
    $\begingroup$ Two paragraphs above formula one in your source (p.1536) , it is described pretty well, imo. $\endgroup$
    – bukwyrm
    Commented Jan 3, 2022 at 10:50
  • 1
    $\begingroup$ You're so right. As soon as I entered my comment I felt like the answer was in what I'd already read. I don't know why it didn't sink in until I read your answer though. Thank you. $\endgroup$
    – Wren
    Commented Jan 3, 2022 at 10:55

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